STEEL HAVING EXCELLENT HYDROGEN-INDUCED CRACKING RESISTANCE AND LOW-TEMPERATURE IMPACT TOUGHNESS, AND METHOD FOR MANUFACTURING SAME

Abstract

The present invention relates to steel suitable for a pressure vessel that can be used as petrochemical manufacturing equipment, a storage tank, and the like, and, more specifically, to steel having excellent hydrogen-induced cracking (HIC) resistance and low-temperature impact toughness and a method for manufacturing same.

Description

TECHNICAL FIELD

The present disclosure relates to a steel material suitable for a pressure vessel that can be used in petrochemical manufacturing equipment, a storage tank, or the like, and more specifically, to a steel material having excellent hydrogen-induced cracking (HIC) resistance and low-temperature impact toughness, and a method for manufacturing the same.

BACKGROUND ART

Recently, pressure vessels used in industries such as mining, production, transportation, storage, refining, and power generation of energy resources are increasing demand for extremely thick steel materials due to an increase in equipment size according to increased usage time. The steel materials require a low carbon equivalent (Ceq) to ensure structural stability of a welded portion. In addition, as production of crude oil containing a large amount of H₂S increases, steel materials for pressure vessels such as the above are required to have hydrogen-induced cracking (HIC) resistance, and as use environment of these structures expands to an extreme cold zone, excellent low-temperature impact toughness may be required at the same time.

A cause of hydrogen-induced cracking (HIC) may be that corrosion occurs when a steel material comes in contact with wet hydrogen sulfide contained in crude oil, and hydrogen atoms generated by the corrosion penetrate and diffuse into the steel material, to have a molecular state as inclusions in the steel material. In this manner, when the hydrogen atoms are moleculized in the steel material, gas pressure may be generated while forming hydrogen gas. Due to this pressure, brittle cracks may occur and grow along a weak structure in the steel material, and breakage may then occur.

Accordingly, as methods for improving the hydrogen-induced cracking resistance of a steel material used in a hydrogen sulfide atmosphere, there may be a method for adding elements such as copper (Cu) and the like, a method for minimizing a hardening structure in which cracks easily occur and propagate, or controlling a shape thereof, a method for controlling internal defects such as inclusions, voids, or the like in a steel material that can act as an initiation point for accumulation and cracking of hydrogen, or other methods.

Patent Document 1 proposes a method of increasing hydrogen-induced cracking resistance by appropriately controlling a shape of a void in a steel material. Specifically, the method was made to form the shape of the void formed in a central portion of the steel material to be as spherical as possible, and a length ratio of a long side and a short side of the void was controlled to be 0.7 or more. However, shapes of voids formed during continuous casting were not constant, and there was a limit to uniformly controlling the shapes by a rolling process, which can lead to deviations in the hydrogen-induced cracking resistance of the steel material, making it necessary to prepare improvement measures.

Meanwhile, as an operating temperature of a steel material for a pressure vessel decreases, impact toughness may decrease, causing stability problems. In particular, as a thickness of a steel material having the same strength increases, toughness of a structure therein may deteriorate to a greater extent. Therefore, to prevent deterioration of impact toughness even at a low temperature, it is necessary to properly manage a composition and a microstructure of a steel material for a pressure vessel applied in regions having low temperature environments.

A rolling process may be one of representative methods of grain refinement. When rolling is performed at a temperature that allows for recrystallization, new austenite fine grains may be created using internal stress generated by rolling force as driving force.

However, as a thickness of a steel material increases, a roll separation force that can be applied by rolling may be limited. Accordingly, it may be difficult to form fine grains by rolling as it is close to an internal structure, especially a central portion of the steel material. As a temperature increases at Ae3 or higher, grains of austenite tend to grow at a higher temperature and for longer heating time, and some alloy elements may have an effect of suppressing the growth of grains of austenite. The alloy elements may be dissolved in steel, and may act as obstacles to grain growth. Therefore, in an extremely thick steel material in which grain refinement is difficult using rolling, the addition of such alloy elements should be considered for grain refinement.

In addition, in quenching and tempering (QT) materials, it is common to perform water cooling and tempering by reheating to a single phase range of austenite after rolling and air-cooling. When a temperature during the reheating is too high or the reheating time is long, the grains of austenite may grow rapidly to deteriorate low-temperature impact toughness. Accordingly, to solve this problem, a technology that can secure excellent low-temperature impact toughness is required.

• • (Patent Document 1) Korean Registered Patent No. 10-2164116

SUMMARY OF INVENTION

Technical Problem

An aspect of the present disclosure relates to a steel material used in a hydrogen sulfide atmosphere, and is to provide a steel material having excellent hydrogen-induced cracking resistance and low-temperature impact toughness, and a method for manufacturing the same.

An object of the present disclosure is not limited to those mentioned above. The additional problems of the present disclosure may be described throughout the specification, and those skilled in the art will have no difficulty in understanding the additional problems of the present disclosure from those described in the specification of the present disclosure.

Solution to Problem

According to an aspect of the present disclosure, a steel material having excellent hydrogen-induced cracking resistance and low-temperature impact toughness, includes, by weight, C: 0.12 to 0.18%, Si: 0.2 to 0.5%, Mn: 0.8 to 1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.005 to 0.025%, Ni: 0.01 to 0.5%, Mo: 0.01 to 0.12%, V: 0.005 to 0.03%, Ti: 0.003% or less (excluding 0), N: 0.002 to 0.01%, Ca: 0.0005 to 0.004%, and a remainder of Fe and inevitable impurities,

• • wherein the number of one or more oxidizing inclusion among an Al—O-based oxidizing inclusion, a Ca—O-based oxidizing inclusion, and an Al—Ca—O-based oxidizing inclusion, having a size of 10 μm or more in the steel material, is 50 or less per 1 mm², and
  • the steel material satisfies the following [Relationship 1] and [Relationship 2]:

$\begin{array}{cc}\mathrm{Ceq}\le 0.45& \left[\mathrm{Relationship}1\right]\end{array}$

Where Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15, and C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components.

$\begin{array}{cc}1.2\le \mathrm{Ca}/S\le 4.& \left[\mathrm{Relationship}2\right]\end{array}$

Where Ca and S are amounts (% by weight) of respective components.

According to another aspect of the present disclosure, a method of manufacturing a steel material having excellent hydrogen-induced cracking resistance and low-temperature impact toughness, includes heating a steel slab including, by weight, C: 0.12 to 0.18%, Si: 0.2 to 0.5%, Mn: 0.8 to 1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.005 to 0.025%, Ni: 0.01 to 0.5%, Mo: 0.01 to 0.12%, V: 0.005 to 0.03%, Ti: 0.003% or less (excluding 0), N: 0.002 to 0.01%, Ca: 0.0005 to 0.004%, and a remainder of Fe and inevitable impurities, and satisfying the following Relationship 1 and Relationship 2 to a temperature range of 1100 to 1200° C.;

• • rough-rolling the heated steel slab at a temperature of 1050° C. or higher and finishing hot-rolling at a temperature of Ar3 or higher after the rough-rolling, to produce a hot-rolled steel sheet;
  • air-cooling the hot-rolled steel sheet;
  • reheating the air-cooled hot-rolled steel sheet to a temperature of Ac3 or higher and maintaining the reheating for (2.3t+30) minutes or more (where t is a thickness of the steel sheet (mm));
  • quenching the reheated hot-rolled steel sheet to room temperature at a cooling rate of 0.4° C./s or more; and
  • tempering the cooled hot-rolled steel sheet for (3.4t+30) minutes or more (where t is a thickness of the steel sheet (mm)) in a temperature range of 600 to 700° C.:

$\begin{array}{cc}\mathrm{Ceq}\le 0.45& \left[\mathrm{Relationship}1\right]\end{array}$

Where Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15, and C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components.

$\begin{array}{cc}1.2\le \mathrm{Ca}/S\le 4.0& \left[\mathrm{Relationship}2\right]\end{array}$

Where Ca and S are amounts (% by weight) of respective components.

Advantageous Effects of Invention

According to an aspect of the present disclosure, a steel material for a pressure vessel having excellent hydrogen-induced cracking resistance and low-temperature impact toughness, after quenching and tempering (QT) and post-welding heat treatment (PWHT), may be provided.

Various advantages and effects of the present disclosure are not limited to those described above, and can be more easily understood through descriptions of specific embodiments of the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates ultrasonic inspection results of evaluating hydrogen-induced cracking of Inventive Example 1 in an embodiment of the present disclosure.

FIG. 2 illustrates ultrasonic inspection results of evaluating hydrogen-induced cracking in Comparative Example 1 in an example of the present disclosure.

BEST MODE FOR INVENTION

Hereinafter, terms used in the present specification are for describing the present disclosure, and are not intended to limit the present disclosure. Additionally, as used herein, singular forms include plural forms unless relevant definitions clearly indicate the contrary.

The meaning of “including” or “comprising” used in the specification specifies a configuration, and does not exclude the presence or addition of another configuration.

Unless otherwise defined, all terms, including technical and scientific terms, used in the present specification have the same meaning as would be commonly understood by a person of ordinary skill in the technical field to which the present disclosure pertains. Terms defined in the dictionary may be interpreted to have meanings consistent with related technical literature and current disclosure.

The present inventors recognized that, as a pressure vessel that can be used in petrochemical industry equipment, storage tanks, or the like increases in terms of a size thereof, is used in a hydrogen sulfide atmosphere, and use environment expands to an extreme cold zone, a method is required to secure properties required for a material thereof. In particular, in a steel material for a pressure vessel having a certain thickness or more, a methods for securing low-temperature impact toughness as well as hydrogen-induced cracking resistance have been studied in depth. As a result, it was confirmed that it was possible to provide a steel material for a pressure vessel having target properties by controlling a composition and relationship between some components in alloy design and optimizing manufacturing conditions, leading to completion of the present disclosure.

An embodiment of a steel material of the present disclosure will be described in detail. First, an alloy composition of the steel material of the present disclosure will be described in detail. The steel material of the present disclosure may comprise, by weight, C: 0.12 to 0.18%, Si: 0.2 to 0.5%, Mn: 0.8 to 1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.005 to 0.025%, Ni: 0.01 to 0.5%, Mo: 0.01 to 0.12%, V: 0.005 to 0.03%, Ti: 0.003% or less (excluding 0), N: 0.002 to 0.01%, Ca: 0.0005 to 0.004%, and a remainder of Fe and inevitable impurities.

Additionally, at least one of Cu: 0.5% or less or Cr: 0.35% or less may be further included.

Carbon (C): 0.12 to 0.18% by weight (hereinafter, referred to as %, unless specifically mentioned in the present disclosure, an amount of each element is based on weight %.)

C may be an element effective in improving strength of steel. To sufficiently achieve this effect, C may be included in an amount of 0.12% or more. When an amount thereof exceeds 0.18%, a degree of segregation in a central portion of the steel may increase, and a martensite-austenite (MA) structure may be formed, which may significantly reduce hydrogen-induced cracking resistance and low-temperature impact toughness. Therefore, it may be advisable not to exceed 0.18%. More advantageously, 0.15% or less may be included.

Silicon (Si): 0.2 to 0.5%

Si may not only be used as a deoxidizing agent, but may be also an element advantageous for improving strength and toughness of steel. To sufficiently obtain this effect, Si may be included in an amount of 0.2% or more. When an amount thereof exceeds 0.5%, there may be a risk of excessive formation of MA, resulting in poor hydrogen-induced cracking resistance and low-temperature impact toughness. Therefore, Si may be in an amount of 0.2 to 0.5%.

Manganese (Mn): 0.8 to 1.5%

Mn may be an element advantageous for improving strength of steel by a solid solution strengthening effect. To fully obtain the effect, Mn may be included in an amount of 0.8% or more. When an amount thereof exceeds 1.5%, it combines with sulfur (S) in the steel to form MnS, which may significantly reduce hydrogen-induced cracking resistance and low-temperature impact toughness. Therefore, Mn may be included in an amount of 0.8 to 1.5%, and more advantageously, may be included in an amount of 1.0 to 1.5%.

Phosphorus (P): 0.15% or Less

P may be an element advantageous in improving strength of steel and securing corrosion resistance thereof, but may greatly impair impact toughness of the steel. Therefore, it is desirable to limit an amount thereof as low as possible. In the present disclosure, even when P is included at a maximum of 0.015%, there may be no difficulty in securing target properties. Therefore, an amount thereof is limited to be 0.015% or less. Considering a level to be unavoidably added, 0% may be excluded.

Sulfur (S): 0.003% or Less

S may be an element that greatly inhibits the hydrogen-induced cracking resistance and impact toughness of steel by combining with Mn in the steel to form MnS and the like. Therefore, it is advantageous to manage S in a low amount as possible. In the present disclosure, even when S is included at a maximum of 0.003%, there may be no difficulty in securing target properties. Therefore, an amount thereof is limited to be 0.003% or less. Considering a level to be unavoidably added, 0% may be excluded.

Aluminum (Al): 0.015 to 0.045%

Al may be an element that may inexpensively deoxidize molten steel. To sufficiently obtain the above-mentioned effect, Al may be included in an amount of 0.015% or more. When an amount thereof exceeds 0.045%, nozzle clogging may occur during continuous casting. This may be undesirable because not only does it cause damage, but impact toughness may be significantly reduced due to formation of Al-based oxidizing inclusions. Therefore, Al may be included in 0.015 to 0.045%.

Niobium (Nb): 0.005 to 0.025%

Nb may precipitate to form NbC or Nb(C,N), greatly improving strength of a base material, and when reheated at high temperature, dissolved Nb may suppress recrystallization of austenite and transformation of ferrite or bainite to obtain a structure refinement effect. For this purpose, Nb may be included in an amount of 0.005 or more. When an amount thereof is excessive, undissolved Nb may form TiNb(C,N), which causes UT defects, and becomes factors of suppressing hydrogen-induced cracking resistance and low-temperature impact toughness. Therefore, it is preferable not to exceed 0.025%. More advantageously, Nb may contain 0.007 to 0.02%.

Nickel (Ni): 0.01 to 0.5%

Ni may be an element that can simultaneously improve strength of a base material and low-temperature impact toughness, and to sufficiently obtain this effect, Ni may be included in an amount of 0.01% or more. Ni may be an expensive element, and when an amount thereof exceeds 0.5%, economic efficiency may be greatly reduced. Therefore, Ni may be included in an amount of 0.01 to 0.5%.

Molybdenum (Mo): 0.01 to 0.12%

Mo may be an element that may be advantageous for greatly improving hardenability of steel and thus strength thereof. To sufficiently achieve this effect, Mo may be included in an amount of 0.01% or more. Mo may be an expensive element, and when an amount thereof is excessive, there may be a risk of inhibiting formation of ferrite and inhibiting low-temperature impact toughness by forming bainite. Therefore, taking this into consideration, it is preferable not to exceed 0.12%.

Vanadium (V): 0.005 to 0.03%

V may have a low solid solution temperature, as compared to other alloy elements, and may have an effect of preventing a decrease in strength by precipitating in a weld heat-affected zone during welding. When strength of a steel material such as that of the present disclosure is not sufficiently secured after post-welding heat treatment (PWHT), a strength improvement effect can be obtained by including 0.005% or more of V. When an amount thereof exceeds 0.03%, a fraction of a hard phase such as MA may increase. Therefore, hydrogen-induced cracking resistance and low-temperature impact toughness may decrease significantly.

Titanium (Ti): 0.003% or Less (Excluding 0%)

When Ti may be added together with N to form TiN, thereby reducing occurrence of surface cracks due to formation of AlN precipitates. When an amount thereof exceeds 0.003%, coarse TiN may be formed during reheating of a steel slab, QT heat treatment, or PWHT process, which may act as a factor impairing low-temperature impact toughness. Therefore, Ti may be in an amount of 0.003% or less.

Nitrogen (N): 0.002 to 0.01%

It may be advantageous that N may be included together with Ti to form TiN and suppress grain growth due to thermal effects during welding. To sufficiently obtain the above-described effects when adding Ti, N may be included in an amount of 0.002% or more. When an amount thereof exceeds 0.01%, it is undesirable because coarse TiN is formed and low-temperature impact toughness is impaired. Therefore, N may be in an amount of 0.002 to 0.01%.

Calcium (Ca): 0.0005 to 0.004%

When added to molten steel, Ca may combine with S to form MnS inclusions, to suppress production of MnS, and may form spherical CaS at the same time, thereby suppressing occurrence of cracks due to hydrogen-induced cracking. To obtain the above-mentioned effect, Ca may be included in an amount of 0.0005% or more. When an amount thereof exceeds 0.004%, a portion of Ca remaining after forming CaS may combine with oxygen (O) to form coarse oxidative inclusions, which may be stretched and destroyed during rolling, which promotes hydrogen-induced cracking. Therefore, Ca may be included in an amount of 0.0005 to 0.004%.

Additionally, in addition to the above composition, one or more of copper (Cu): 0.5% or less and chromium (Cr: 0.35% or less) may be further included.

Copper (Cu): 0.5% or Less

Cu may be an element that may greatly improve strength by solid solution strengthening, and may be an element that effectively suppresses corrosion of a base material in a wet hydrogen sulfide atmosphere. In a strong acid atmosphere, the above effect may not be significant, and when an amount of Cu is excessive, it may not only impair weldability due to an increase in carbon equivalent, but also significantly deteriorate surface quality of a product. Therefore, when adding Cu, it may be included at a maximum of 0.5%. In the present disclosure, there may be no difficulty in securing target properties even when Cu is not added. Therefore, it is noted that Cu is not essential.

Chromium (Cr): 0.35% or Less

Cr may be an element that can prevent strength decline by slowing down a decomposition rate of cementite during tempering or post-welding heat treatment (PWHT). When an amount thereof exceeds 0.35%, coarse carbides may increase and impact toughness may be greatly reduced. Therefore, Cr may include a maximum of 0.35%. In the present disclosure, there may be no difficulty in securing target properties even when Cr is not added. Therefore, it is noted that Cr is not essential.

The remainder may include iron (Fe) and inevitable impurities. Inevitable impurities may be unintentionally mixed in the normal steel manufacturing process, and, thus, may not be completely excluded, and any engineer in the normal steel manufacturing field can easily understand meaning thereof. In addition, the present disclosure does not completely exclude addition of compositions, other than the steel compositions mentioned above.

To secure hydrogen-induced cracking resistance and low-temperature impact toughness as well as a target level of strength in a steel material of the present disclosure, it is desirable to appropriately adjust amounts of elements advantageous for improving properties by adding a certain amount thereof. Therefore, a carbon equivalent (Ceq) of the following Relationship 1 may be 0.45 or less. When the carbon equivalent (Ceq) exceeds 0.45, it is advantageous in securing strength, but there may be a risk that properties after welding may be greatly impaired. In addition, when large amounts of alloy elements are included, the costs will increase and economic feasibility will be impaired. Therefore, the carbon equivalent (Ceq) may be 0.45 or less.

$\begin{array}{cc}\mathrm{Ceq}\le 0.45& \left[\mathrm{Relationship}1\right]\end{array}$

Where Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15, and C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components.

In addition, the steel material of the present disclosure may satisfy the following Relationship 2:

$\begin{array}{cc}1.2\le \mathrm{Ca}/S\le 4.0& \left[\mathrm{Relationship}2\right]\end{array}$

Where Ca and S are amounts (% by weight) of respective components.

When Ca/S is less than 1.2, MnS may be formed instead of CaS, which can significantly increase impact toughness and hydrogen-induced cracking in a central portion. When Ca/S is more than 4, a complex inclusion of CaO—Al₂O₃ and CaS may be formed and may also cause inferiority in impact toughness and hydrogen-induced cracking. Therefore, Ca/S may be 1.2 to 4.0.

A microstructure of the steel material may have an area fraction of polygonal ferrite of 70% or more, an area fraction of pearlite of 20 to 30%, and the remainder may be bainite (including 0%). When the area fraction of polygonal ferrite is less than 70%, impact toughness may decrease significantly, and when the area fraction of pearlite exceeds 20 to 30%, strength may be reduced or exceeded.

An average grain size of the polygonal ferrite may be 25 μm or less. When the average grain size of the polygonal ferrite exceeds 25 μm, impact toughness may decrease significantly.

The steel material may include an Al—O-based oxidizing inclusion, a Ca—O-based oxidizing inclusion, an Al—Ca—O-based oxidizing inclusion, or the like, having a size of 10 μm or more therein, may be 50 or less per 1 mm². The oxidizing inclusions of which size is less than 10 μm may not have a significant effect on properties, and therefore may not have a significant technical significance. When the number of the oxidizing inclusions exceeds 50/mm², occurrence of hydrogen organic cracking may increase with a high probability.

There may be no significant difference in the microstructural characteristics of the steel described above before and after post-welding heat treatment (PWHT), which will be described later.

The steel may have an average crack length ratio (CLR) value of 10% or less of an experiment performed under NACE TM0284 Solution A (strong acid) conditions, which is a related international standard, from a reference surface to a central portion, based on a width central portion.

The steel may have a yield strength of 260 MPa or more, a tensile strength of 485 MPa or more, and an average Charpy impact absorption energy (CVN, −46° C.) value of 150J or more at a temperature of −46° C., when evaluated perpendicular to a rolling direction at a t/4 point in a thickness direction (where t is a thickness (mm) of the steel material), providing excellent strength and low-temperature impact toughness.

Physical properties of the steel material described above may be physical properties of steel material that has undergone post-welding heat treatment (PWHT) on the steel material.

Next, an embodiment of a method for manufacturing a steel material of the present disclosure will be described in detail. The manufacturing method may include heating a steel slab that satisfies the above-described alloy composition, and manufacturing the same by hot-rolling, cooling, reheating, quenching, and tempering.

Heating of Steel Slab

Homogenization treatment may be performed by heating a steel slab satisfying an alloy composition, as described above. In this case, it is desirable to perform heating at a temperature range of 1100 to 1200° C. When a heating temperature of the steel slab is less than 1100° C., a precipitate (carbide, nitride) formed in the slab may not be sufficiently re-dissolved, thereby reducing formation of precipitate in a process after hot-rolling. When the slab extraction temperature exceeds 1200° C., a grain of austenite may coarsen and properties of the steel may deteriorate.

Hot-Rolling

A hot-rolled steel sheet may be manufactured by hot-rolling the heated steel slab as described above. Rough-rolling may be performed on the heated steel slab at a temperature of 1050° C. or higher, and then finish hot-rolling may be performed at a temperature of Ar3 or higher.

When the temperature during rough-rolling is less than 1050° C., there may be a problem in that the temperature may decrease during subsequent finishing hot-rolling. In this case, it may be important to prevent grains from coarsening by providing sufficient reduction force during rough-rolling. Therefore, it is desirable to provide a reduction ratio of 10% or more in the last pass of rough-rolling. When the rolling force may not be sufficient during rough-rolling, there may be a high possibility that the grains will become coarse after rough-rolling.

In addition, when the finishing hot-rolling temperature is lower than Ar3, the rolling load may increase, and there may be a risk of quality defects such as surface cracks.

Ar3 can be expressed as follows.

$\mathrm{Ar}3=910-310C-80\mathrm{Mn}-20\mathrm{Cu}-55\mathrm{Ni}-80\mathrm{Mo}+119V+124\mathrm{Ti}-18\mathrm{Nb}+179\mathrm{Al}$

Where, each element is an amount (% by weight)).

Cooling and Reheating

After air-cooling is performed on the hot-rolled steel sheet manufactured as above to room temperature, reheat the same to a temperature of Ac3 or higher, and maintain the same for a certain period of time. The reheating process can induce creation of a fine austenite structure and contribute to the refinement of ferrite after water cooling. The hot-rolled steel sheet can be reheated to form an austenite structure, but when the reheating temperature is lower than Ac3, there may be a risk that the hot-rolled steel sheet structure will have a two-phase structure of ferrite and austenite. Therefore, the reheating may be performed at a temperature range of Ac3 or higher, preferably 830 to 930° C., and the reheating may be performed at the temperature for (2.3t+30) minutes or more (where t is a thickness of the steel (mm). When the holding time is less than (2.3t+30) minutes, 100% austenizing may not be achieved due to insufficient holding time, and there may be a risk that tensile and impact toughness will be greatly reduced due to abnormal heat treatment. Since an upper limit of the holding time has no physical meaning, it may not be particularly limited, and a person skilled in the art can easily determine the same considering equipment limitations or the like.

Ac3 can be expressed as follows.

$\mathrm{Ac}3=93.2-436.5C+56\mathrm{Si}-19.7\mathrm{Mn}-26.6\mathrm{Ni}+38.1\mathrm{Mo}+124.8V+136.3\mathrm{Ti}-19.1\mathrm{Nb}+198.4\mathrm{Al}$

Where, each element is an amount (% by weight)).

Quenching and Tempering

It may be desirable to quench the reheated hot-rolled steel sheet to room temperature at a cooling rate of 0.4° C./s or more. When the cooling rate during cooling is less than 0.4° C./s, the microstructure may include coarsened ferrite and pearlite phases, which may impair strength and low-temperature impact toughness.

Tempering may be performed on the cooled hot-rolled steel sheet at a temperature range of 600 to 700° C. for more than (3.4t+30) minutes or more (where t is a thickness of the steel (mm)). When the cooled hot-rolled steel sheet is heat treated at a temperature below 600° C., it may be difficult to form fine precipitates, making it difficult to secure strength, and when the temperature exceeds 700° C., hydrogen-induced cracking resistance and low-temperature impact toughness may be greatly deteriorated due to formation of coarse precipitates. When the tempering time is less than (3.4t+30) minutes, strength may be secured by heat treatment at a temperature lower than the target temperature due to insufficient tempering time, but there may be a risk that impact toughness may increase. Since an upper limit of the tempering time has no technical meaning, it may not be particularly limited, and a person skilled in the art can easily determine the same considering equipment limitations or the like.

Cooling after the tempering is not particularly limited, but may be performed by air-cooling.

Welding may be performed on the steel manufactured as above, and post-welding heat treatment (PWHT) may be performed. In general, since steel materials for pressure vessels are used by welding, it is common to perform PWHT heat treatment to overcome toughness deterioration of a welded zone. In the present disclosure, the welding and PWHT processes are not particularly limited. For example, it may be necessary to stabilize the toughness after welding by subjecting the steel to PWHT heat treatment for 1 hour or more per inch of steel thickness in the temperature range of 550 to 650° C.

When the temperature during the PWHT heat treatment is less than 550° C., long-term heat treatment may be required, which may reduce economic feasibility. When the temperature exceeds 650° C., not only will a strength reduction effect become excessively large, but there may be also a risk that impact toughness may also decrease due to coarsening of carbides.

A steel material on which the PWHT heat treatment has been completed may be air-cooled to room temperature, and a steel material composed of ferrite, pearlite, and the remaining bainite phase can be obtained.

MODE FOR INVENTION

Next, examples of the present disclosure will be described.

Various modifications to the following examples may be made by those skilled in the art without departing from the scope of the present disclosure. The following examples are for understanding of the present disclosure, and the scope of the present disclosure should not be limited to the following examples, but should be determined by the claims described below as well as their equivalents.

EXAMPLE

A slab was manufactured by continuously casting molten steel having the alloy composition (% by weight, the remainder being Fe and inevitable impurities) illustrated in Table 1 below. In this case, the slab was manufactured to have a thickness of 700 mm (Hereinafter, IE: Inventive Example, CE: Comparative Example, and R: Relationship).

	TABLE 1
	C	Si	Mn	P	S	Al	Nb	Cu	Cr	Ni	Mo	V	Ti	Ca	N	R1	R2
IE1	0.160	0.370	0.950	0.010	0.001	0.30	0.010	0.250	0.250	0.350	0.100	0.010	0.002	0.0015	0.0035	0.430	1.5
IE2	0.140	0.370	1.450	0.010	0.001	0.030	0.017	—	—	0.350	0.100	0.007	0.002	0.0016	0.0035	0.426	1.6
IE3	0.150	0.350	1.250	0.008	0.001	0.030	0.016	0.100	0.150	0.300	0.100	0.007	0.002	0.0017	0.0035	0.436	1.7
IE4	0.155	0.370	1.400	0.009	0.001	0.030	0.017	—	—	0.250	0.110	0.006	0.002	0.0015	0.0034	0.428	1.5
IE5	0.160	0.250	1.050	0.008	0.001	0.030	0.013	0.100	0.100	0.300	0.080	0.010	0.002	0.0015	0.0030	0.400	1.5
CE1	0.160	0.350	1.100	0.009	0.001	0.030	0.003	0.005	0.150	0.250	0.090	0.007	0.002	0.0003	0.0035	0.410	0.3
CE2	0.185	0.370	1.000	0.010	0.001	0.030	0.017	0.100	0.100	0.350	0.100	0.010	0.002	0.0018	0.0033	0.424	1.8
CE3	0.145	0.370	1.450	0.010	0.001	0.050	0.016	—	—	0.300	0.100	0.008	0.002	0.0020	0.0033	0.428	2.0

In Table 1 above, Relationships 1 and 2 may be calculated as follows:

$\begin{array}{cc}\mathrm{Ceq}\le 0.45& \left[\mathrm{Relationship}1\right]\end{array}$

Where Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15, and C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components.

$\begin{array}{cc}1.2\le \mathrm{Ca}/S\le 4.0& \left[\mathrm{Relationship}2\right]\end{array}$

Where Ca and S are amounts (% by weight) of respective components.

The continuous slab was reheated to about 1000° C. or higher, forged to a thickness of about 400 mm, and then air-cooled.

The forged slab was heated to about 1100° C., rough-rolling was performed at about 1050° C. or higher, and then finish hot-rolling was performed at about 980° C., to obtain a hot-rolled steel sheet having a thickness of about 200 mm.

The hot-rolled steel sheet was air-cooled to room temperature, reheated to about 890° C., maintained for about 480 minutes, water-cooled (quenched) to room temperature, reheated to about 650° C., held for about 710 minutes (tempering), and then air-cooled to perform QT heat treatment. Afterwards, the air-cooled hot-rolled steel sheet was heated to about 635° C., maintained for about 1,200 minutes to perform a post-weld heat treatment (PWHT), and then air-cooled to room temperature to manufacture a final steel material. The detailed conditions are illustrated in Table 2.

	TABLE 2
	Slab	Slab	Finish		Reheating		Tempering
	Extraction	Heating	Rolling	Reheating	Holding	Tempering	Holding
	Temp.	Time	Temp.	Temp .	Time	Temp.	Time
	(° C.)	(min.)	(° C.)	(° C.)	(min.)	(° C.)	(min.)
IE1	1111	361	980	890	505	655	731
IE2	1110	362	981	891	499	651	730
IE3	1109	360	980	890	506	650	729
IE4	1110	359	981	891	506	654	728
IE5	1108	356	982	890	506	655	730
CE1	1112	359	979	888	507	654	731
CE2	1110	360	985	889	509	653	732
CE3	1109	361	980	890	507	651	734

A microstructure and mechanical properties of a steel material manufactured as above were evaluated. The microstructure was observed using an optical microscope, and then fraction and diameter of ferrite were measured using an analysis program. In this case, the microstructure was measured at a point t/4 (t may be the steel thickness, mm) in the thickness direction of each steel material, and the results were illustrated in Table 3 below.

In addition, the mechanical properties were evaluated at a ¼t point in a thickness direction of each steel material. In this case, tensile specimens were collected from each thickness direction point in a direction, perpendicular to a rolling direction, to measure tensile strength (TS), yield strength (YS), and elongation (El) was measured, and the impact specimen was taken from a JIS No. 4 standard test specimen at a ¼t point in the thickness direction in the rolling direction, and the average impact toughness (CVN) at −46° C. was measured. The results were illustrated in Table 3 below.

The inclusions refer to an Al—O-based oxidizing inclusion, a Ca—O-based oxidizing inclusion, an Al—Ca—O-based oxidizing inclusion, or the like, having a size of 10 μm or more.

A hydrogen-induced cracking crack length ratio (CLR, %) in the longitudinal direction of the sheet, used as an indicator of the hydrogen-induced cracking resistance of steel sheets, was evaluated by immersing the specimen in 5% NaCl+0.5% CH₃COOH solution, saturated with H₂S gas, for 96 hours at 1 atm according to the relevant international standard NACE TM0284. The length of the crack was measured by ultrasonic inspection, measuring a total length of each crack in the longitudinal direction of the specimen, and dividing a total length of each crack in the length direction of the specimen by a total length of the specimen, and results therefrom were illustrated in Table 3.

	TABLE 3
		Number of
	Microstructure (area %)	Oxidative	Tensile Properties	−46° C. CVN
	Polygonal			Inclusions	YP	TS	El.	Impact	CLR
	Ferrite	Pearlite	Bainite	(Number/mm2)	(MPa)	(MPa)	(%)	Toughness(J)	(%)
IE 1	78	22	0	12	347	497	37	251	0
IE 2	70	30	C	21	358	508	36	275	0
IE 3	72	22	6	17	355	499	36	246	0
IE 4	73	20	7	15	345	500	36	228	0.6
IE 5	74	22	4	22	338	502	37	255	0.3
CE1	78	22	0	28	321	480	38	241	10.5
CE2	65	20	15	24	368	532	32	84	21.5
CE3	71	29	0	86	355	504	37	198	16.4

As illustrated in Table 3, Inventive Steels 1 to 5 manufactured according to the alloy composition, component relationships, and manufacturing conditions, proposed in the present disclosure, satisfied a microstructure, tensile properties, low-temperature impact toughness, and hydrogen-induced cracking resistance proposed in the present disclosure.

It can be seen that, in Comparative Example 1, amounts of Nb and Ca were outside the range proposed in the present disclosure, and not only tensile strength was low due to lack in amount of Nb, but also a Ca/S ratio was outside the value suggested in the present disclosure, a CLR value deviated from the value suggested in the present disclosure due to insufficient control of MnS. It can be seen that Comparative Example 2 was a component system in which an amount of C was outside the range presented in the present disclosure, and tensile properties can be sufficiently secured, but low-temperature impact toughness may be outside the value presented in the present disclosure, and the CLR value may also increase due to an increase in the hard phase. It can be seen that, in Comparative Example 3, most of the components satisfied the values suggested in the present disclosure, but an amount of Al was too high, and although tensile and low-temperature impact toughness were satisfied, due to presence of Al-based oxides, it may act as a starting point of hydrogen-organic cracks occurred, and the CLR value may greatly deviate from the value suggested in the present disclosure.

FIGS. 1 and 2 illustrate results of ultrasonic inspection of test pieces after the hydrogen-induced cracking tests at a ½t point in a central portion of a width for each of the steel materials of Invention Example 1 and Comparative Example 1. It can be seen that, in Inventive Example 1 of FIG. 1, no hydrogen-induced cracking occurred at all, while Comparative Example 1 of FIG. 2, in which Nb and Ca were outside the values suggested in the present disclosure, hydrogen-induced cracking occurred.

Claims

1. A steel material having excellent hydrogen-induced cracking resistance and low-temperature impact toughness, comprising: Ceq ⁢ ≤ 0. 4 ⁢ 5 [ Relationship ⁢ 1 ] 1.2 ≤ Ca / S ≤ 4. 0 [ Relationship ⁢ 2 ]

by weight, C: 0.12 to 0.18%, Si: 0.2 to 0.5%, Mn: 0.8 to 1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.005 to 0.025%, Ni: 0.01 to 0.5%, Mo: 0.01 to 0.12%, V: 0.005 to 0.03%, Ti: 0.003% or less (excluding 0), N: 0.002 to 0.01%, Ca: 0.0005 to 0.004%, and a remainder of Fe and inevitable impurities,

wherein the number of one or more oxidizing inclusion among an Al—O-based oxidizing inclusion, a Ca—O-based oxidizing inclusion, and an Al—Ca—O-based oxidizing inclusion, having a size of 10 μm or more in the steel material, is 50 or less per 1 mm2, and

the steel material satisfies the following [Relationship 1] and [Relationship 2]:

where Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15, and C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components,

where Ca and S are amounts (% by weight) of respective components.

2. The steel material of claim 1, further comprising at least one of Cu: 0.5% or less or Cr: 0.35% or less.

3. The steel material of claim 1, wherein a fraction of ferrite is 70% or more, a fraction of pearlite is 20 to 30%, and the remainder is bainite (including 0%).

4. The steel material of claim 3, wherein an average grain size of the ferrite is 25 μm or less.

5. The steel material of claim 1, wherein, when the steel material is evaluated perpendicular to a rolling direction at a t/4 point in a thickness direction (where t is a thickness (mm) of the steel material), after welding and heat treatment (PWHT) of the steel material, a yield strength is 260 MPa or more, a tensile strength is 485 MPa or more, and an average Charpy impact absorption energy (CVN, −46° C.) value at a temperature of −46° C. is 150J or more.

6. A method of manufacturing a steel material having excellent hydrogen-induced cracking resistance and low-temperature impact toughness, comprising: Ceq ⁢ ≤ 0. 4 ⁢ 5 [ Relationship ⁢ 1 ] 1.2 ≤ Ca / S ≤ 4. 0 [ Relationship ⁢ 2 ]

heating a steel slab including, by weight, C: 0.12 to 0.18%, Si: 0.2 to 0.5%, Mn: 0.8 to 1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.005 to 0.025%, Ni: 0.01 to 0.5%, Mo: 0.01 to 0.12%, V: 0.005 to 0.03%, Ti: 0.003% or less (excluding 0), N: 0.002 to 0.01%, Ca: 0.0005 to 0.004%, and a remainder of Fe and inevitable impurities, and satisfying the following Relationship 1 and Relationship 2 to a temperature range of 1100 to 1200° C.;

rough-rolling the heated steel slab at a temperature of 1050° C. or higher and finishing hot-rolling at a temperature of Ar3 or higher after the rough-rolling, to produce a hot-rolled steel sheet;

air-cooling the hot-rolled steel sheet;

reheating the air-cooled hot-rolled steel sheet to a temperature of Ac3 or higher and maintaining the reheating for (2.3t+30) minutes or more (where t is a thickness of the steel sheet (mm));

quenching the reheated hot-rolled steel sheet to room temperature at a cooling rate of 0.4° C./s or more; and

tempering the cooled hot-rolled steel sheet for (3.4t+30) minutes or more (where t is a thickness of the steel sheet (mm)) in a temperature range of 600 to 700° C.:

where Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15, and C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components,

where Ca and S are amounts (% by weight) of respective components.

7. The method of claim 6, wherein the steel slab further comprises at least one of Cu: 0.5% or less or Cr: 0.35% or less.

8. The method of claim 6, comprising performing post-weld heat treatment (PWHT) for more than 1 hour per inch of steel thickness in a temperature range of 550 to 650° C., after welding the steel material.

9. The method of claim 7, comprising performing post-weld heat treatment (PWHT) for more than 1 hour per inch of steel thickness in a temperature range of 550 to 650° C., after welding the steel material.